This invention relates generally to gaseous fuel internal combustion engines. More particularly, this invention concerns a novel stratified charge combustion apparatus and method for use in such engines.
Conventional spark ignited internal combustion engines burn a homogeneous mixture of air and fuel supplied to the combustion chamber from a mixing device, such as a carburetor. To achieve ignition of this homogeneous mixture, the air-to-fuel ratio must be maintained within a relatively narrow range, which prevents optimum combustion of the air/fuel mixture over the full range of operating levels. For example, if the engine is operating at a low power output, the air intake must be throttled to reduce the amount of air taken into the combustion chamber, thereby maintaining the air-to-fuel ratio within the ignitable range. Throttling the air intake, however, results in increased friction horsepower and increased pumping losses, thereby reducing engine efficiency.
In contrast, by concentrating fuel in the vicinity of the ignition source, stratified charge internal combustion engines maintain the air-to-fuel ratio near the ignition source within the ignitable range. Combustion is thus possible in a stratified charge combustion system even though the overall air-to-fuel ratio may run well outside the ignitable limits. The resulting oxygen-rich environment of the combustion chamber produces more complete combustion of the fuel. The stratified charge combustion engine thus can run more efficiently and also exhausts less emissions than conventional internal combustion engines.
Additionally, because the air-to-fuel ratio is high in those portions of the combustion chamber remote from the ignition source, stratified charge combustion systems alleviate the problem of detonation. This permits the utilization of higher compression ratios compared to compression ratios of standard homogeneous mixture internal combustion engines. Higher compression ratios, in turn, yield lower fuel consumption, and thus will lower engine operating costs and conserve energy.
The advantages of stratified charge internal combustion have long been known and practiced in liquid fuel internal combustion engines. Different methods have been used to create a fuel rich mixture in the vicinity of the ignition source. For example, U.S. Pat. No. 3,318,292 to Hideg discloses various embodiments of a liquid fuel stratification charge system. Particularly, several of the described embodiments illustrate a liquid fuel injector that directs the liquid fuel against a wall of the combustion chamber where it is vaporized and subsequently transported to the ignition source by the turbulent motion of combustion air in the combustion chamber. In another embodiment, a liquid fuel is injected against at least one surface of an air intake port, intake passage, or intake valve where it is vaporized and carried into the combustion chamber by the incoming air.
Another method of creating liquid fuel charge stratification is shown in U.S. Pat. No. 3,911,873 to Dave. This method involves a variable internal combustion engine valve operating system, one embodiment of which illustrates a shrouded valve for directing the flow of a carbureted fuel/air mixture toward a spark plug.
A further example is shown in U.S. Pat. No. 3,154,059 to Witzky et al., which discloses an injection system in which liquid fuel is injected near the periphery and against the direction of swirling air in the combustion chamber to create a rich fuel/air mixture near the ignition source. Liquid fuel is directed against the cylinder head to evaporate it.
U.S. Pat. No. 3,809,039 to Alquist, also directed to stratified charge liquid fuel internal combustion engines, discloses a spark ignition system in which a rich fuel/air mixture is passed into a precombustion chamber where it is spark ignited. The ignited rich fuel/air mixture then induces ignition of a lean fuel/air mixture in the main combustion chamber.
In present commercial gaseous fuel internal combustion engines, the gaseous fuel is introduced into the combustion chamber in one of two ways. First, the combustion air and gaseous fuel may enter the combustion chamber together through either intake valves or intake ports. Second, the combustion air may be admitted separately through either intake valves or intake ports. The gaseous fuel is then separately injected into the combustion chamber by a gaseous fuel injection valve. Typically, the valve is non-directional; that is, the gaseous fuel is injected 360.degree. around the valve, creating a roughly cone-shaped dispersion of the gaseous fuel in the combustion chamber. Occasionally, a directional fuel injection valve is used to inject the gaseous fuel at less than 180.degree. around the valve.
Dual chamber combustion systems, illustrated by the Fairbanks Morse M.E.P. Model 81/8 Spark Ignited Energy Cell Gaseous Fuel Engine, have been used in gaseous fuel internal combustion engines. A rich homogeneous fuel/air mixture is spark ignited in a precombustion chamber. This ignited rich fuel/air mixture then induces ignition of a lean fuel/air mixture in the main combustion chamber. This type of combustion system is sometimes referred to as a stratified charge system.
In all these commercially available gaseous fuel engines, however, the fuel injection system is designed to achieve a homogeneous mixture of the gaseous fuel and the combustion air in the main combustion chamber at the time of ignition. Even when a separate gaseous fuel injection valve is used, location of valves in the combustion chamber, the directional orientation of the valves, and the angle of the valves is designed to direct the gaseous fuel toward the center of the combustion chamber and/or into the mass of combustion chamber air. Thus, present gaseous fuel internal combustion engines possess many of the disadvantages of homogeneous charge liquid fuel internal combustion engines, including inefficient fuel comsumption, which results in higher fuel costs and produces greater amounts of air pollutants.
In summary, charge stratification has been successfully applied in commercial liquid fuel engines but not in commercial gaseous fuel engines. The charge stratification systems used in liquid fuel engines cannot be applied to gaseous fuel engines because of the extreme differences between gas behavior and liquid behavior.
The differences in gas and liquid behavior begin with the differences in their physical properties. Liquid and gas fluid characteristics are so different that they each have separate points of reference. Water is generally used as a reference for liquids, whereas air is used for gases. See Marks' Standard Handbook for Mechanical Engineers, p. 3.varies.35 (8th ed.). Viscosity of liquids is orders of magnitude greater than the viscosity of gases. Liquids have surface tension, and gases do not. Also, liquids have cohesion between their molecules and the ability to adhere to a surface, whereas gases do not. See Marks' Standard Handbook for Mechanical Engineers, pps. 3-36 to 3-37 (8th ed.).
These differences in properties have a direct impact on the art of fuel injection. Liquid fuels tend not to break up as they flow into the combustion chamber air. The liquid will stream through the air until it loses its velocity and impacts a solid surface. By contrast, a gaseous fuel will naturally tend to break up and immediately mix with the combustion air when it enters the chamber. The gaseous fuel is not able to penetrate the air mass to any great extent because of its low inertia (mass and velocity).
This ability of an injected liquid fuel to retain a defined jet while a gaseous fuel breaks up is graphically depicted in the Society of Automotive Engineers Technical Paper Series No. 830590 entitled "An Exprimental and Analytical Study of Flash-Boiling Fuel Injection", FIGS. 8 and 16 at pages 8 and 11.
Further, a liquid fuel will stick to a surface of the combustion chamber after impact because of its viscosity and surface tension characteristics. By contrast, a gas will not cling to a surface without the proper fluid dynamics of impact angle, surface contour, and speed to hold the gas on the surface.
A liquid fuel in its liquid state will not ignite or burn. By contrast, a gaseous fuel will ignite and burn. Therefore, liquid fuels must be vaporized before they can be ignited. See the Society of Automotive Engineers Technical Paper Series No. 830587 entitled "Analysis of Hydrocarbon Emissions Mechanisms in a Direct Injection Spark-Ignition Engine", FIG. 21, page 21. As previously mentioned, both U.S. Pat. No. 3,318,292 to Hideg and U.S. Pat. No. 3,154,059 to Witzky et al., both dealing with liquid fuel engines, teach the necessity for vaporization before ignition in a liquid fuel engine.
Although small droplets of a liquid can be carried in suspension in air, they will not mix with the air. Only after the liquid has been vaporized will it mix with air. Once a mixture is obtained, the vaporized fuel can be separated from the air only be reliquifying or condensing. By contrast, a gaseous fuel is already a vapor at injection and will immediately mix with the air in the combustion chamber.
Finally, the energy content of a given volume of liquid fuel is many times greater than the energy content of an equal volume of gaseous fuel. For example, a cubic foot of natural gas contains about 1,000 BTUs. A cubic foot of gasoline contains approximately 1,000,000 BTUs, a ratio of 1,000:1. Due to the extremely large differences in the energy content of a volume of liquid fuel as compared to a gaseous fuel, the actual volume of liquid fuel injected per combustion cycle is minute as compared to the large volume of gaseous fuel injected into an internal combustion engine of the same size and power. The difference in energy per volume between a liquid and a gaseous fuel is due to the weight per volume difference between the two types of fuels.
The liquid fuel, with its much higher weight per volume, can be pinpointed in its injection into the cylinder, since the volume of liquid is small relative to the volume of combustion air or the size of the cylinder. By contrast, gaseous fuel with its extremely low weight to volume ratio has a natural tendency to cover the total volume of the cylinder as it is injected.
Accordingly, the properties and resulting behavior of liquid and gaseous fuels are so divergent that injection of one into a combustion chamber is entirely different from injection of the other. For example, both U.S. Pat. No. 3,318,292 to Hideg and U.S. Pat. No. 3,154,059 to Witzky et al. teach directing the liquid fuel into the volume of the combustion chamber air and using the swirling motion of the combustion air to transport the vaporized fuel to the ignition source. Of course, if this method were used in a gaseous fuel engine, too much mixing of the gaseous fuel with the combustion chamber air would take place, and no stratification could be achieved.
As another example, if a liquid fuel were directed toward an ignition source without mixing with and being vaporized by the combustion chamber air, the wet liquid would cover the ignition source and the fuel would not ignite. As previously stated, the liquid fuel must first be vaporized before ignition can occur.
Because of the natural tendency of gases to diffuse, the gaseous fuel must be injected into the combustion chamber so as to reach the ignition source with minimal mixing of the gaseous fuel and the combustion chamber air. Only enough mixing should occur to provide enough air in the fuel/air mixture at the ignition source to support combustion.
Canadian Pat. No. 869,305 to Ward et al discloses a two cycle gaseous fuel engine with a large, non-directional, differential pressure operated gaseous fuel valve that depends solely on the scavaging air flow pattern for distribution of the fuel-air mixture in the combustion chamber at the time of ignition. Air flow from the inlet ports loops upwardly in aspirating flow across the chamber, allowing a check valve to open by differential pressure, which draws the gaseous fuel into the combustion chamber. The looping air flow draws the cloud of gaseous fuel downwardly along the wall of the cylinder above the exhaust port. Then when the piston moves upwardly, it compresses the air and gaseous fuel trapped in the cylinder and the fuel cloud moves upwardly toward a glow plug. It is believed that this type of system permits too much mixing of the air and gaseous fuel for optimum charge stratification. As long as the gaseous fuel is directed into the body of the combustion chamber air mass, too much mixing of the gaseous fuel with the air will occur.